1. Field of the Invention
The present invention relates to a variable displacement pump employed for a power steering device or the like of a vehicle.
2. Description of the Related Art
A variable displacement pump is disclosed in Japanese Patent Application Laid-open (JP-A) No. 8-200239, to assist steering force by means of a hydraulic power steering device for a vehicle. This conventional variable displacement pump is directly rotated and driven by means of an engine of the vehicle. A rotor is provided in a cam ring engaged movably and displaceably with an adapter ring engaged with a pump casing, and forms a pump chamber between the cam ring and the periphery of the rotor.
In this conventional technology, there is provided an ejection flow rate control device in which the cam ring is movable and displaceable in the adapter ring. Biasing force is imparted to the cam ring by a spring such that a capacitance of the pump chamber is maximized. First and second fluid pressure chambers are formed to be divided between the cam ring and the adapter ring. The cam ring is moved against the biasing force by a differential pressure acting to both of the fluid pressure chambers. The capacitance of the pump chamber is changed, thereby making it possible to control an ejection flow rate. In this manner, in this variable displacement pump, the ejection-flow rate is increased so that a large steering assistance force can be obtained during stoppage or low-speed running of a vehicle with its low rotation frequency. The ejection flow rate is controlled to a predetermined quantity so that the steering assistance force is reduced during high-speed running with its high rotation frequency, whereby the steering assistance force required for the power steering device can be generated.
Hence, in the conventional technology, the adapter ring is annular. When this adapter ring is engaged with the pump casing, the adapter ring is thin, and thus, it cannot be pressed-in. Therefore, the adapter ring is forced to have a slight gap between the adapter ring and the pump casing during engagement with the pump casing. The presence of this gap causes unwanted vibration of the adapter ring during pump actuation, and causes abnormal noise to be generated.
It is an object of the present invention to reduce in a variable displacement pump the generation of vibration and abnormal noise of the adapter ring during pump actuation while the assembling properties of engagement of the adapter ring with the pump casing is improved in a variable displacement pump.
According to the present invention, there is disclosed a variable displacement pump comprising:
a rotor fixed to a pump shaft inserted into a pump casing to be rotatably driven, the rotor housing a number of vanes in grooves and being movable in a radial direction;
an adapter ring engaged with an engagement hole of the pump casing;
a cam ring engaged with the adapter ring and forming a pump chamber between the cam ring and the periphery of the rotor;
an ejection flow rate control device making it possible to move and displace the cam ring in the adapter ring, imparting to the cam ring a biasing force such that the capacitance of a pump chamber is maximized, forming first and second fluid pressure chambers to be divided between the cam ring and the adapter ring, moving the cam ring against said biasing force with a differential pressure acting to both of the fluid pressure chambers, and changing the capacitance of the pump chamber, thereby making it possible to control an ejection flow rate.
A slit is provided over the widthwise area for the adapter ring partially in the peripheral direction of said adapter ring. The outer diameter in a free state of the adapter ring is greater than a hole diameter of an engagement hole of said pump casing, and the adapter ring is engaged with the pump casing while resilient diameter expandability is imparted to the adapter ring to bring it into close contact with the engagement hole.